The widely used CTG equipment for monitoring of fetal heart activity apply ultrasound Doppler technique, where an ultrasonic beam is directed to the fetus, the reflection of which determines the heart movement and hereby fetal heart rate (FHR). However, these equipment are unsuitable for long-time monitoring without inspection, since uncontrolled high-dose exposure may have some harmful effects.
A further technique for this purpose is the phonocardiography (PCG) where acoustic waves excited by heart movement are detected. In the case of fetal heart sounds, however, difficulties arise in the detection and identification of the signals. Acoustic waves of fetal origin reach the sensor placed on the maternal abdomen through a complex transmission path, where significant spectrum variation occurs. Furthermore, disturbances of maternal digestive organs may hinder signal detection. Finally, fetus movements result in the displacement of the optimal sensing point and lead to the decrease of signal level.
An essential advantage of the acoustic method is that the passive manner of the sensing is harmless to the fetus even at very long monitoring time, which makes possible home care. The detection and processing of acoustic signals of fetal heart are dealt with by many researchers.
A fetal monitor is announced in U.S. Pat. No. 2,536,527 to Appel. The invention serves for monitoring fetal condition during delivery. A microphone applied to the stethoscope produces a signal which is amplified, filtered, rectified and used to indicate abnormally high or low FHRs.
U.S. Pat. No. 3,187,098 to Farrar describes a fetal heartbeat detector, which uses a cantilevered piezoelectric crystal mounted within a contacting slab. A fetal monitor is given in U.S. Pat. No. 3,409,737 to Settler et al. This monitor is used with a belt having three microphones. An amplifier is used to selectively amplify the fetal heartbeat and remove the maternal heartbeat.
U.S. Pat. No. 4,781,200 to Baker uses a sensor belt wearing twelve sensors, the detected signal of which are compared to cancel disturbances and trace fetal position. The signal processing is carried out by the conventional FFT method for 128 points applied in every 1/8 seconds, delivering the frequency spectra for selection of coincided components of the sensors. However, because of the relatively long time period of analysis the fast variations in spectral power density are averaged. Thus short time characteristics of frequency components cannot be correctly detected using this method. U.S. Pat. No. 5,140,992 to Zuckerwar et al. uses a belt wearing more piezoelectric polymer film sensors for fetal heartbeat indication.
All of this invention have the common insufficiency that they do not distinguish perfectly the first and second sound and thus they cannot apply this additional information to the identification of fetal heartbeat sound. In addition, a lot of computations are carried out in signal processing, which can be saved without the deterioration of the reliable sound identification. As a consequence, these instruments require high supply current and thus they are for battery operation unsuitable.
Reliable identification of fetal heartbeat is of vital importance at FHR measurement. Consequently, there is need for a solution that enables reliable identification of fetal heartbeats, does not require bulky hardware and allows battery operation even without medical supervision.